The present invention is directed to X-Y-Z data collection devices and systems. The present invention is directly applicable to many important surface inspection tasks and Reverse-CAD functions that have not been successfully accomplished in the prior art because of data collection speeds, mechanical unreliability and equipment costs.
In the prior art X-Y-Z data collection systems have been traditionally of the single-point contact probe type associated with Coordinate Measuring Machines (CMM). These CMM systems involve extensive and cumbersome X-Y-Z mechanical motions to place the probe in contact with the part to generate a single X-Y-Z data point. The speed limitations of these systems involve the motions of the mechanical units which are required to accelerate and decelerate between points in order for a light contact of the probe to be accomplished. Efforts to improve speeds have resulted in more complex and more expensive mechanical units in order to avoid the effects of mechanical wear, shock and other reliability factors. Advances in probe technology has also resulted in the development of single-point, non-contact probes based on the principles of triangulation. Although the probes no longer come in contact with the part, the extensive mechanical motions are still retained in order to collect the part data.
Additional advances in X-Y-Z data systems include laser line scanning techniques such as that described in U.S. Pat. No. 4,895,434. In these systems, a vision sensor such as a CCD video camera views an area of the part. A laser line at an offset angle is mechanically swept through the viewing area of the camera and a number of images are collected for data processing in order to generate the X-Y-Z surface information. This unit requires a finite amount of time for the camera to be held stationary with respect to the part while the laser line is mechanically scanned and the images are collected. After the data is collected, typically in one or more seconds, a positioning unit is used to either reposition the part relative to the camera or vise versa. A further advancement in sensor technology has eliminated the requirement for the mechanical scanning of the laser line. In particular, the EOIS MK VII product manufactured by Electro-Optical Information Systems, Inc. includes a projection of an array of linear lines, i.e. a linear fringe pattern onto the part for viewing by the camera. In this configuration at least two fringe patterns need to be projected sequentially to permit the camera to collect X-Y-Z data through an adequate optical depth-of-field. The second pattern can be generated by either mechanically changing the fringe pattern in the optical projector or by using a second projector to cast the pattern. This second fringe pattern is in sequence to the primary fringe pattern and requires that the camera be held stationary with respect to the part for approximately a fraction of a second. In all of these applications, a mechanical positioner is required to step between the points or fields-of-view of the cameras with the requirements of high accuracy acceleration and deceleration which create further time delays.
At the system level, the methods of improving overall functional speeds involve a "thinning" of data for less data collection. This method, however, has made the data collection systems less usable for surface inspection applications in which detailed surface characteristics and flaws are to be measured and in Reverse-CAD operations in which extensive surface details are to be measured and stored.
One of the most significant problems in high-speed 3-D surface mapping sensors, in which fixed projection patterns are used, is the ability to generate an X-Y-Z surface map which is not only accurate over a large Z range (for nomenclature convenience, this is the axis approximately normal to the surface under measure), but which is also a high spatial resolution in X and Y (the local axes approximately tangent to the surface under measure). The prior art which includes shadow moire (U.S. Pat. Nos.: 3,627,427; 4,577,940; 4,525,858; and 4,939,380), projection moire (U.S. Pat. Nos.: 4,212,073; 3,943,278; 4,850,693; and 4,874,955), projected fringe moire (U.S. Pat. Nos.: 4,070,683; 4,867,570; and 4,842,411), does not effectively solve this problem. The fundamental methods of phase-shift moire, which have a potentially high X-Y spatial resolution, are not suitable as the grating patterns need to be shifted several times over a fringe period causing associated time delays. This is because the part needs to be held stationary during the relatively long collection interval. Fixed fringe pattern moire (whether of the shadow type, projected type, with or without camera optics reference gratings) can also achieve good surface map X-Y spatial resolution and Z accuracies if the fringe pattern is of high density. In principle, the spatial resolution in the direction of the fringes (the Y-axis) is the same as the pixel size of the CCD imaging camera. Across the fringe pattern (the X-axis) the spatial resolution is determined by the Nyquest limit established by the highest fundamental frequency of the projected pattern (which could include compound fringe patterns of several fundamental frequencies). As long as fringe pattern image contrast can be maintained in the optical system, a number of measurement advantages occur as the fringe pattern becomes more dense. These are: (1) greater surface map spatial resolution in the X-axis, (2) greater measurement sensitivity and resolution in the Z-axis, and (3) smaller measurable X-Y surface patch areas. The disadvantage of these high density patterns is that the dynamic range of the Z-axis measurement is severely limited by the fringe pattern spacing. As the surface height changes in Z, the fringe pattern shifts in the camera field-of-view. When the fundamental frequency of the fringe pattern shifts one full cycle, the information on surface location is lost. This is known as the "2.pi. Problem" in moire and interferometric fringe pattern processing and limits the Z-axis dynamic range of measurement to approximately the spacing of one fringe pattern cycle. A number of techniques are used to counter this problem including those of the referenced inventions, but each solution detracts from the goal of the present invention to achieve high-speed X-Y-Z surface measurement and mapping while achieving the previously identified advantages to high density fringes.
A summary of previously identified techniques to solve the 2.pi. problem and Z-axis dynamic range problem follows. One class of techniques is to project sequentially different fringe patterns to eliminate the 2.pi. ambiguity. However this has the same problem as the phase-shift moire techniques in that a finite time interval is required to collect the data and to hold the part stationary with respect to the sensor. For fixed pattern techniques, a common solution is to code the fringes, or to superimpose additional lower frequency fringe patterns onto the fundamental fringe pattern, or to add special "identification" artifacts to the pattern to remove the 2.pi. ambiguity. Each of these techniques increases the Z measurement dynamic range but reduces the performance of the "stand-alone" high density fringe pattern. In order to maintain the Z accuracies of the measurements with these techniques, a more extensive image filtering process in required to reduce the potential noise or errors induced by the modified pattern features. This translates directly into requiring that the measurable X-Y surface patches to be larger than otherwise required. Thus the usefulness of these techniques are significantly reduced when high-speed 3-D measurements of small dimensional features on parts are required such as corner radii, edge contours, fillets, corrugation, serrations, etc.